1. Field of the Invention
This invention relates to an automatic focusing device for the lens system having lenses of the zooming function.
2. Description of the Prior Art
From the past, as the zoom lens of such type there has been known, for example, the rear focus type. Also in the zoom lenses of such type, when the focal length is changed by zooming, the sharpness of an image for the same object formed by this lens is caused to change. In the camera or the like, therefore, the image on the film plane is blurred. To avoid this, the position of the focusing member is corrected by an automatic focusing device of the TTL type to bring the image into focus, as has already been known, for example, in U.S. Pat. No. 3,972,056.
Here, about the property of such type zoom lens is explained, taking an example of the zoom lens of the rear focus type, by reference to FIG. 1. FIG. 1 shows the relationship between the preset focal length and the required distance the focusing lens moves from the position for infinity with the parameter in the object distance, wherein the ordinate is in the focal length of the entire system and the abscissa is in the movement of the focusing lens. As is obvious from this or first figure, in such type zoom lens, with object at infinity, the in-focus position of the focusing lens remains constant for both of the wide angle and telephoto ends of a zooming range. But as the object distance decreases, the in-focus position of the focusing lens for the telephoto end becomes different from that for the wide angle side. These characteristics depend on the smoothly varying curves of second degree. Also, conversely it will be understood from this graph of FIG. 1 that if the focusing lens is left stationary, the image plane is deviated by zooming. The above-cited example of the prior art is based on the idea that the deviation of the image plane is detected by using an automatic focusing device, and the focusing lens is automatically driven to obtain always a sharp image on the film plane.
By the way, with the use of the commonly available automatic focusing device, as the image on the film plane in the camera blurs largely, when it becomes impossible to determine which direction the focusing lens must move to for the image is brought into focus, the focusing lens is let wander before the in-focus position is reached, or the operating mechanism for the focusing lens is stopped from further driving it. The above-described prior art system has, therefore, the following drawback. When it happens that the zooming operation is carried out at so fast a speed that the image becomes very unsharp, it will take a long time to establish the in-focus condition of the focusing lens, and a good shutter chance will be missed.